Zoom lens

Abstract

A zoom lens includes, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power, wherein, during zooming from a wide-angle end to a telephoto end, at least the first, third, fourth and fifth lens units are axially moved toward the object side in such a manner that a separation between the first lens unit and the second lens unit increases and a separation between the second lens unit and the third lens unit decreases, and wherein the fifth lens unit is axially moved toward the image side to effect focusing on from an infinitely distant object to a closest object.

I claim: 
     
       1. A zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power, and a fifth lens unit of negative refractive power, wherein, during zooming from a wide-angle end to a telephoto end, at least said first, third, fourth, and fifth lens units are axially moved independently toward the object side in such a manner that a separation between said first lens unit and said second lens unit increases and a separation between said second lens unit and said third lens unit decreases, and   wherein only said fifth lens unit is axially moved toward the image side to effect focusing from an infinitely distant object to a closest object.   
     
     
       2. A zoom lens according to claim 1, satisfying the following conditions:   2.0<f1/fW<6.0       0.3<|f2/fW|<0.85       0.9<f3/fW<2.3       |β4W|<0.5       1≦β5W<2.5       β5W<β5T     where f1, f2 and f3 are focal lengths of said first, second and third lens units, respectively, fW is a focal length in the wide-angle end of the entire zoom lens, β4W and β5W are lateral magnifications in the wide-angle end of said fourth lens unit and said fifth lens unit, respectively, and β5T is a lateral magnification in the telephoto end of said fifth lens unit.   
     
     
       3. A zoom lens according to claim 1 or 2, wherein said fifth lens unit comprises a negative lens of meniscus form convex toward the object side and a positive lens of meniscus form convex toward the object side. 
     
     
       4. A zoom lens according to claim 3, wherein said negative lens and said positive lens of said fifth lens unit are cemented together. 
     
     
       5. A zoom lens according to claim 4, satisfying the following condition:   ν52<ν51     where ν51 and ν52 are Abbe numbers of materials of said negative lens and said positive lens of said fifth lens unit, respectively.   
     
     
       6. A zoom lens according to claim 1 or 2, wherein said first lens unit comprises a cemented lens composed of a negative lens of meniscus form convex toward the object side and a positive lens, and a positive lens of meniscus form convex toward the object side, and wherein said second lens unit comprises a negative lens of meniscus form convex toward the object side, a negative lens of bi-concave form, a positive lens of bi-convex form, and a negative lens having a concave surface facing the object side. 
     
     
       7. A zoom lens according to claim 1 or 2, wherein said third lens unit comprises a positive lens of bi-convex form, a cemented lens composed of a positive lens of bi-convex form and a negative lens, and a cemented lens composed of a positive lens and a negative lens. 
     
     
       8. A zoom lens according to claim 1, wherein said fourth lens unit comprises at least two positive lenses and at least one negative lens. 
     
     
       9. A zoom lens according to claim 1 or 2, wherein said fourth lens unit comprises a positive lens having a convex surface facing the image side, a negative lens of meniscus form convex toward the image side, and a positive lens of bi-convex form. 
     
     
       10. A zoom lens according to claim 1 or 2, wherein said fifth lens unit comprises a negative lens of meniscus form convex toward the object side and a positive lens of meniscus form convex toward the object side. 
     
     
       11. A zoom lens according to claim 1, wherein said fifth lens unit comprises a negative lens of meniscus form convex toward the object side, a positive lens of meniscus form convex toward the object side, a negative lens of meniscus form convex toward the object side, and a positive lens of meniscus form convex toward the object side.